Steel sheet for hot pressing and method for manufacturing hot-pressed member using the same

ABSTRACT

A steel sheet for hot pressing that is capable of suppressing formation of scales or ZnO in hot pressing and excellent in oxidation resistance, and a method for manufacturing a hot-pressed member using the steel sheet are provided. The steel sheet for hot pressing includes a base steel sheet and a plating layer that is formed on a surface of the base steel sheet at a coating weight of 10 to 90 g/m 2  and contains 10 to 25% by mass of Ni and the balance Zn with inevitable impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase application ofPCT/JP2011/076636, filed Nov. 18, 2011, and claims priority to JapanesePatent Application No. 2010-261928, filed Nov. 25, 2010, Japanese PatentApplication No. 2011-000554, filed Jan. 5, 2011, Japanese PatentApplication No. 2011-091634, filed Apr. 18, 2011, Japanese PatentApplication No. 2011-162680, filed Jul. 26, 2011, the disclosures ofeach of these applications being incorporated herein by reference intheir entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a steel sheet for hot pressing suitablefor manufacturing members, such as suspension members or body structuralmembers of an automobile, by hot pressing and a method for manufacturinga hot-pressed member using the steel sheet for hot pressing.

BACKGROUND OF THE INVENTION

Conventionally, many members such as suspension members or bodystructural members of an automobile are manufactured by pressing a steelsheet that has predetermined strength. In recent years, from a viewpointof global environmental conservation, the weight saving of an automobilebody is desired, and efforts are being made to reduce the steel-sheetthickness by increasing steel-sheet strength. However, because the pressworkability of steel sheet is deteriorated with an increase in thestrength of the steel sheet, it is difficult to form the steel sheetinto a desired shape in many cases.

Patent Literature 1 discloses a pressing technique called hot pressing.This technique realizes both easy pressing and high strength byquenching a heated steel sheet at the same time when the heated steelsheet is pressed by using a mold formed of a die and a punch. However,in this hot pressing, because the steel sheet is heated to the hightemperature of around 950° C. before hot pressing, scales (iron oxide)are formed on the surface of the steel sheet. The scales formed on thesurface of the steel sheet are peeled off during hot pressing and hence,the mold is damaged or the surface of the member is damaged after hotpressing. Furthermore, the scales remaining on the surface of the membercause a defect in appearance and a defect in painting adhesion.Accordingly, the scales formed on the surface of the member are usuallyremoved by pickling or shot-blasting. These additional processes makemanufacturing processes complicated thus causing deterioration inproductivity. Furthermore, the members such as suspension members andbody structural members of the automobile also require excellentanti-corrosion characteristics. However, an anti-corrosive coating suchas a plating layer is not applied to hot-pressed members manufactured inthe above-mentioned processes and hence, the anti corrosioncharacteristics of the members are extremely insufficient.

In such a background, a hot pressing technique capable of suppressingformation of the scales during heating before hot pressing and improvingthe anti-corrosion characteristics is required. Therefore, a steel sheetto which the anti-corrosive coating such as the plating layer is appliedand a method for hot-pressing a member using the steel sheet areproposed. For example, Patent Literature 2 discloses a manufacturingmethod for a hot-pressed member having excellent anti-corrosioncharacteristics. This manufacturing method includes a step ofhot-pressing a steel sheet coated with Zn or Zn-based alloy and a stepof providing a Zn—Fe-based compound or a Zn—Fe—Al-based compound on thesurface of the heated steel sheet.

PATENT LITERATURE

-   Patent Literature 1: UK Patent No. 1490535-   Patent Literature 2: Japanese Patent No. 3663145

SUMMARY OF THE INVENTION

In the hot-pressed member manufactured by the method described in PatentLiterature 2, there exists a case that the defect in appearance and thedefect in painting adhesion due to the formation of the scales arecaused, and the degradation of the anti corrosion characteristics due tothe generation of ZnO is caused.

For the foregoing reasons, the present invention provides a steel sheetfor hot pressing and a method for manufacturing a hot-pressed memberusing the steel sheet for hot pressing, wherein the steel sheet isexcellent in oxidation resistance and capable of suppressing theformation of the scales and ZnO in hot pressing.

The inventors have acquired knowledge described below as a result ofintensive examinations on a steel sheet for hot pressing.

(1) Scales are liable to be formed on a local portion, such as adefective portion of a plating layer, or cracks produced by hot pressingfrom a portion (starting point of cracks) on which Zn—Fe metalliccompound are formed during heating.

(2) Scales or ZnO is liable to be formed on the Zinc-plating layerhaving a melting point lower than 700° C.

(3) In order to suppress formation of the scales or ZnO, it is effectivethat the plating layer contains 10 to 25% by mass of Ni having a highmelting point and the balance Zn with inevitable impurities.

According to an embodiment of the present invention, there is provided asteel sheet for hot pressing including: a base steel sheet; and aplating layer that is formed on a surface of the base steel sheet at acoating weight of 10 to 90 g/m² and contains 10 to 25% by mass of Ni andthe balance Zn with inevitable impurities.

It is preferable that the steel sheet for hot pressing further includesat least one compound layer selected from the group consisting ofSi-containing compound layer, Ti-containing compound layer,Al-containing compound layer, and Zr-containing compound layer, whereinthe compound layer is formed on the plating layer.

The base steel sheet, which is a steel sheet before the plating layer isprovided and is a base for the plating layer, may have componentcomposition having 0.15 to 0.5% by mass of C, 0.05 to 2.0% by mass ofSi, 0.5 to 3% by mass of Mn, 0.1% by mass or less of P, 0.05% by mass orless of S, 0.1% by mass or less of Al, 0.01% by mass or less of N, andthe balance Fe with inevitable impurities. It is preferable that thebase steel sheet further contains at least one component selected fromthe group consisting of 0.01 to 1% by mass of Cr, 0.2% by mass or lessof Ti, and 0.0005 to 0.08% by mass of B. It is preferable that the basesteel sheet further contains, in addition to/separately from theabove-mentioned at least one component, 0.003 to 0.03% by mass of Sb.

According to an embodiment of the present invention, there is provided amethod for manufacturing a hot-pressed member, the method including:heating the above-mentioned steel sheet for hot pressing to atemperature ranging from Ac₃ transformation point to 1000° C.; andhot-pressing the heated steel sheet.

According to the present invention, it is possible to manufacture thesteel sheet for hot pressing that is capable of suppressing formation ofthe scales or ZnO in hot pressing and excellent in oxidation resistance.A hot-pressed member manufactured by a method for manufacturing thehot-pressed member in the present invention with the use of the steelsheet for hot pressing in the present invention is excellent inappearance, and has excellent painting adhesion and anti-corrosioncharacteristics. Hence, the hot-pressed member is suitable for asuspension member or a body structural member of an automobile.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION (1) Steel Sheet forHot Pressing

(1-1) Plating Layer

In an embodiment of the present invention, in order to suppressformation of scales or ZnO in hot pressing, a plating layer containing10 to 25% by mass of Ni and the balance Zn with inevitable impurities isprovided on a surface of a steel sheet. A content of Ni contained in theplating layer is set to 10 to 25% by mass whereby a γ phase having anyone crystal structure of Ni₂Zn₁₁, NiZn₃, or Ni₅Zn₂₁ and having a highmelting point of 881° C. is formed. Accordingly, in heating, theformation of the scales or ZnO can be minimized. Furthermore, duringheating, Zn—Fe-based metallic compound is not formed and hence, theformation of the scales accompanying with production of cracks issuppressed. Furthermore, the plating layer having such compositionremains on the steel sheet as the γ phase even after completion of hotpressing thus exhibiting excellent anti-corrosion characteristics by asacrificial protection effect of Zn. Here, in the case where the contentof Ni is 10 to 25% by mass, the formation of the γ phase does notcorrespond with data in the equilibrium diagram for Ni—Zn alloy. Thisresult may be brought about by the fact that a plating-layer formingreaction that is performed by an electroplating method or the likeproceeds in a state of non-equilibrium. The γ phases of Ni₂Zn₁₁, NiZn₃,or Ni₅Zn₂₁ can be confirmed by an electron diffraction method with theuse of an X-ray diffraction method or TEM (Transmission ElectronMicroscopy). Furthermore, although the γ phase is, as described above,formed by setting Ni content contained in the plating layer to 10 to 25%by mass, there exists a case that some quantity of a η phase is alsoformed depending on the electroplating conditions or the like. In thiscase, in order to minimize the formation of the scales or ZnO duringheating, it is preferable to set an η phase content to 5% by mass orless. The η phase content is defined as the ratio of the weight of the ηphase to the total weight of the plating layer. For example, the η phasecontent can be determined by the anodic dissolution method or the like.

The coating weight of the plating layer is set to 10 to 90 g/m² for oneside. When the coating weight of the plating layer is less than 10 g/m²,the sacrificial protection effect of Zn is not sufficiently exhibited.When the coating weight of the plating layer is more than 90 g/m², thesacrificial protection effect of Zn becomes saturated thus increasingthe cost of manufacturing.

Although a method for forming the plating layer is not particularlylimited, it is preferable to use a well-known electroplating method.

At least one compound layer selected from the group consisting of aSi-containing compound layer, a Ti-containing compound layer, anAl-containing compound layer and a Zr-containing compound layer isfurther provided on the plating layer thus obtaining excellent paintingadhesion. In order to obtain such an effect, it is preferable to set athickness of the compound layer to 0.1 μm or more. Furthermore, it ispreferable to set the thickness of the compound layer to 3.0 μm or less.When the thickness of the compound layer exceeds 3.0 μm, the compoundlayer becomes brittle thus causing the defect in painting adhesion. Thethickness of the compound layer is more preferably within the range from0.4 to 2.0 μm.

As a Si-containing compound, for example, silicone resin, lithiumsilicate, silicate soda, colloidal silica, and a silane coupling agentare applicable. As a Ti-containing compound, for example, titanate suchas lithium titanate or calcium titanate, and a titanium coupling agentcontaining titanium alkoxide or a chelate-type titanium compound as abase compound are applicable. As an Al-containing compound, for example,aluminate such as sodium aluminate or calcium aluminate, and an aluminumcoupling agent containing aluminum alkoxide or a chelate-type aluminumcompound as a base compound are applicable. As a Zr-containing compound,for example, zirconate such as lithium zirconate and calcium zirconate,a zirconium coupling agent containing zirconium alkoxide or achelate-type zirconium compound as a base compound are applicable.

In order to form the above-mentioned compound layer on the platinglayer, at least one compound selected from the group consisting of theSi-containing compound, the Ti-containing compound, the Al-containingcompound, and the Zr-containing compound may be adhere to the platinglayer and thereafter, may be baked without being washed with water. Asan adhesion treatment for these compounds, any of an application method,a dipping method, and a spray method may be used. Furthermore, a rollcoater, a squeeze coater, a die coater or the like may be used. Afterperforming an application treatment by the squeeze coater, a dippingtreatment or spraying treatment, it is also possible to adjust theamount of the compound and uniformize appearance and the thickness ofthe compound layer with the use of an air knife method or a squeeze rollmethod. Furthermore, the maximum arriving temperature of the steel sheetwhen baking may be within the range from 40 to 200° C., and preferablywithin the range from 60 to 160° C.

Furthermore, the method for forming the above-mentioned compound layeron the plating layer is not limited to the above-mentioned method. It isalso possible to form the above-mentioned compound layer on the platinglayer by a method described below; for example, a reactive treatmentsuch that the steel sheet having the plating layer is immersed in anacidic aqueous solution containing at least one cation selected from thegroup consisting of Si, Ti, Al, and Zr and at least one anion selectedfrom the group consisting of phosphoric acid ion, fluorine acid ion, andfluoride ion is performed and thereafter, the steel sheet is baked withbeing washed with water or without being washed with water.

The above-mentioned compound layer may contain inorganic solidlubricant. By the addition of the inorganic solid lubricant, thecoefficient of dynamic friction at the time of hot pressing is reducedthus improving the press workability.

As the inorganic solid lubricant, at least one substance selected fromthe group consisting of metal sulfide (molybdenum disulfide, tungstendisulfide or the like), a selenium compound (molybdenum selenide,tungsten selenide or the like), graphite, fluoride (graphite fluoride,calcium fluoride or the like), nitride (boron nitride, silicon nitrideor the like), borax, mica, metallic tin, and alkali metal sulfate(sodium sulfate, potassium sulfate or the like) is applicable. It ispreferable that the content of the above-mentioned inorganic solidlubricant contained in the compound layer is 0.1 to 20% by mass. In thecase where the content of the inorganic solid lubricant is 0.1% by massor more, the compound layer is capable of providing a lubricationeffect. In the case where the content of the inorganic solid lubricantis 20% by mass or less, the painting adhesion is not degraded.

(1-2) Base Steel Sheet

In order to obtain the hot-pressed member having tensile strength (TS)of 980 MPa or more, as a base steel sheet before plating, for example, ahot-rolled steel sheet or a cold-rolled steel sheet including componentcomposition having 0.15 to 0.5% by mass of C, 0.05 to 2.0% by mass ofSi, 0.5 to 3% by mass of Mn, 0.1% by mass or less of P, 0.05% by mass orless of S, 0.1% by mass or less of Al, 0.01% by mass or less of N, andthe balance Fe with inevitable impurities may be used. The reason thatthe content of each component element is limited is explainedhereinafter. Here, “%” expressing the content of the component means “%by mass” unless otherwise specified.

C: 0.15 to 0.5%

The component element C is an element that improves the strength ofsteel. In order to increase the tensile strength of the hot-pressedmember to 980 MPa or higher, it is necessary to set the content of C to0.15% or more. On the other hand, in the case that the content of Cexceeds 0.5%, blanking workability of the steel sheet as a raw materialis remarkably deteriorated. Therefore, the content of C is within therange from 0.15 to 0.5%.

Si: 0.05 to 2.0%

The component element Si is an element that improves the strength ofsteel in the same manner as the case of C. In order to increase thetensile strength of the hot-pressed member to 980 MPa or higher, it isnecessary to set the content of Si to 0.05% or more. On the other hand,in the case that the content of Si exceeds 2.0%, generation of a surfacedefect called red scale is extremely increased and, at the same time, arolling load is increased or ductility of the hot-rolled steel sheet isdeteriorated. Furthermore, in performing a plating treatment such thatthe plating film containing Zn or Al as a main component is formed onthe surface of the steel sheet, there exists a case that the Si contentthat exceeds 2.0% adversely affects plating processability. Therefore,the content of Si is within the range from 0.05 to 2.0%.

Mn: 0.5 to 3%

The component element Mn is an element effective for improvinghardenability by suppressing ferrite transformation, and also effectivefor lowering heating temperature before hot pressing by lowering Ac₃transformation point. In order to exhibit such effects, it is necessaryto set the content of Mn to 0.5% or more. On the other hand, in the casethat the content of Mn exceeds 3%, Mn is segregated, and uniformity ofcharacteristics of the steel sheet as the raw material and thehot-pressed member is lowered. Therefore, the content of Mn is withinthe range from 0.5 to 3%.

P: 0.1% or less

When the content of P exceeds 0.1%, P is segregated, and uniformity ofcharacteristics of the steel sheet as the raw material and thehot-pressed member is lowered and, at the same time, toughness thereofis also lowered. Therefore, the content of P is set to 0.1% or less.

S: 0.05% or Less

When the content of S exceeds 0.05%, toughness of the hot-pressed memberis lowered. Therefore, the content of S is set to 0.05% or less.

Al: 0.1% or Less

When the content of Al exceeds 0.1%, blanking workability orhardenability of the steel sheet as a raw material is deteriorated.Therefore, the content of Al is set to 0.1% or less.

N: 0.01% or Less

When the content of N exceeds 0.01%, the nitride such as AlN is formedduring hot-rolling or heating before hot pressing, and blankingworkability or hardenability of the steel sheet as a raw material isdeteriorated. Therefore, the content of N is set to 0.01% or less.

The balance of the above-mentioned component contained in the base steelsheet is constituted of Fe with the inevitable impurities. However, dueto the following grounds, it is preferable that at least one componentselected from the group consisting of 0.01 to 1% Cr, 0.2% or less Ti,and 0.0005 to 0.08% B, and/or 0.003 to 0.03% Sb are contained in thebalance.

Cr: 0.01 to 1%

The component element Cr is effective for strengthening the steel andimproving hardenability of the steel. In order to exhibit such effects,it is preferable to set the content of Cr to 0.01% or more. On the otherhand, in the case that the content of Cr exceeds 1%, the manufacturingcost for the base steel sheet is considerably increased. Therefore, itis preferable that the content of Cr is set to 1% maximum.

Ti: 0.2% or Less

The component element Ti is effective for strengthening the steel andimproving toughness of the steel by refining crystalline grains.Furthermore, the component element Ti is also effective for formingnitride in priority to the component element B mentioned below andexhibiting the effect of improving hardenability by solid-solvedcomponent element B. However, in the case that the content of Ti exceeds0.2%, a rolling load is extremely increased during hot-rolling and thetoughness of the hot-pressed member is lowered. Therefore, it ispreferable that the content of Ti is set to 0.2% maximum.

B: 0.0005 to 0.08%

The component element B is effective for improving hardenability in hotpressing and toughness after hot pressing. In order to exhibit sucheffects, it is preferable to set the content of B to 0.0005% or more. Onthe other hand, in the case that the content of B exceeds 0.08%, therolling load is extremely increased during hot-rolling and cracks or thelike in the steel sheet is produced due to martensitic phases orbainitic phases formed after hot-rolling. Therefore, it is preferablethat the content of B is set to 0.08% maximum.

Sb: 0.003 to 0.03%

The component element Sb is effective for suppressing a decarburizedlayer formed in the surface layer part of the steel sheet while coolingthe steel sheet in performing a series of processes from the process ofheating the steel sheet before hot pressing to the process of hotpressing. In order to exhibit such an effect, it is necessary to set thecontent of Sb to 0.003% or more. On the other hand, in the case that thecontent of Sb exceeds 0.03%, a rolling load is extremely increasedduring hot-rolling thus decreasing productivity. Therefore, it ispreferable that the content of Sb is within the range from 0.003 to0.03%.

(2) Method for Manufacturing Hot-Pressed Member

The above-mentioned steel sheet for hot pressing is hot-pressed afterbeing heated to a temperature ranging from Ac₃ transformation point to1000° C., thus forming the hot-pressed member. The steel sheet is heatedto the temperature equal to or higher than Ac₃ transformation point forforming a hard phase such as a martensitic phase or the like byquenching in hot pressing to increase the strength of the member.Furthermore, the heating temperature is 1000° C. maximum, because alarge amount of ZnO is formed on the surface of the plating layer whenthe heating temperature exceeds 1000° C. Here, the heating temperaturemeans the maximum arriving temperature of the steel sheet.

Although the average temperature-rising rate upon heating before hotpressing is not limited particularly to a certain rate, for example, theaverage temperature-rising rate ranging from 2 to 200° C./s ispreferable. The amount of ZnO formed on the surface of the plating layerand the amount of scales locally formed on defective portions of theplating layer are increased along with the increase of the hightemperature residence time for which the steel sheet is exposed to ahigh temperature. Therefore, it is preferable to increase the averagetemperature-rising rate because the formation of scales is suppressed.Furthermore, although the holding time at the maximum arrivingtemperature of the steel sheet is also not limited particularly to thecertain time, for the reason same as above, it is preferable to shortenthe holding time. The holding time is preferably set to 300 seconds orshorter, more preferably to 120 seconds or shorter, and furtherpreferably to 10 seconds or shorter.

As a heating method before hot pressing, a heating method with the useof an electric furnace or a gas furnace, a flame heating method, aconduction heating method, a high frequency induction heating method, aninduction heating method or the like can be illustrated.

First Example

As a base steel sheet, cold-rolled steel sheets including componentcomposition having 0.23% by mass of C, 0.25% by mass of Si, 1.2% by massof Mn, 0.01% by mass of P, 0.01% by mass of S, 0.03% by mass of Al,0.005% by mass of N, 0.2% by mass of Cr, 0.02% by mass of Ti, 0.0022% bymass of B, 0.008% by mass of Sb, and the balance Fe with inevitableimpurities, and having Ac₃ transformation point of 820° C. and thethickness of 1.6 mm were used. The electroplating treatment was appliedto the surfaces of the cold-rolled steel sheets by changing currentdensity from 5 to 100 A/dm² in a plating bath with pH 1.5 and 50° C.temperature solution containing 200 g/L of nickel sulfate hexahydrateand 10 to 100 g/L of zinc sulfate heptahydrate. As illustrated in Tables1 to 3, steel sheets Nos. 1 to 19 having plating layers that aredifferent in Ni content (the balance is Zn with inevitable impurities),coating weight, and η-phase content were prepared. Furthermore, forcomparison with the steel sheets Nos. 1 to 19, steel sheets Nos. 20 to24 of a hot dip galvanized steel sheet (GI), a galvannealed steel sheet(GA), a hot dip Zn-5% Al plated steel sheet (GF), and a hot dip Zn-55%Al plated steel sheet (GL) that were made by applying hot-dip plating tothe above-mentioned cold-rolled steel sheets, and a cold-rolled steelsheet with no plating layer were prepared. Tables 1 to 4 shows the thusprepared steel sheets Nos. 1 to 24.

Each of the steel sheets Nos. 1 to 24 was heated in the electric furnaceor by direct energization under the heating condition specified inTables 1 to 4 and, thereafter, cooled at 50° C./s cooling rate in astate that each steel sheet was sandwiched between dies made ofaluminum. Next, oxidation resistance described below was evaluated.Tables 1 to 4 lists compositions of the plating layers, heatingconditions, and evaluation results of the oxidation resistance for eachsteel sheet.

Oxidation resistance: Each steel sheet was heated under the heatingcondition illustrated in Tables 1 to 4 and, thereafter, the weight ofeach steel sheet was measured and the change in weight was measured bycomparing the weight after heating to the weight before heating. Here,the change in weight is obtained by adding the weight increased by theformation of the scales or ZnO and the weight reduced by the scatteringof ZnO formed. The oxidation resistance is enhanced along with thereduction of the absolute value of the change in weight. The oxidationresistance was evaluated on the basis of the following criteria. Whenthe result of the evaluation is “Excellent” or “Good”, the benefit ofthe present invention is achieved.

Excellent: the absolute value of the change in weight ≦3 g/m²

Good: 3 g/m²< the absolute value of the change in weight ≦5 g/m²

Poor: 5 g/m²< the absolute value of the change in weight

As illustrated in Tables 1 to 4, it is evident that the steel sheetsNos. 1 to 16 have small absolute values of the change in weight and areexcellent in oxidation resistance.

TABLE 1 Plating layer Heating conditions Oxidation resistance Kind Coat-Average Heating Hold- Change Oxida- Steel of Ni ing η-phase temperature-temper- ing in tion sheet compo- content weight content rising rateature time weight Judg- appear- Re- No. nent (mass %) (g/m²) (mass %) (°C./s) (° C.) (s) (g/m²) ment ance marks 1 Zn—Ni 12 45 0 30 900 0 +1.5Excel- Present- lent invention example 2 Zn—Ni 10 45 1 30 900 0 +4.0Good Present- invention example 3 Zn—Ni 18 45 0 30 900 0 +2.0 Excel-Present- lent invention example 4 Zn—Ni 25 45 0 30 900 0 +3.5 GoodPresent- invention example 5 Zn—Ni 12 10 0 30 900 0 +4.5 Good Present-invention example 6 Zn—Ni 12 30 0 30 900 0 +2.5 Excel- Present- lentinvention example 7 Zn—Ni 12 60 0 30 900 0 +1.0 Excel- Present- lentinvention example 8 Zn—Ni 12 90 0 30 900 0 +0.5 Excel- Present- lentinvention example 9 Zn—Ni 12 45 0 3 900 0 +4.0 Good Present- inventionexample

TABLE 2 Plating layer Heating conditions Oxidation resistance Kind Coat-Average Heating Hold- Change Oxida- Steel of Ni ing η-phase temperature-temper- ing in tion sheet compo- content weight content rising rateature time weight Judg- appear- Re- No. nent (mass %) (g/m²) (mass %) (°C./s) (° C.) (s) (g/m²) ment ance marks 10 Zn—Ni 12 45 0 10 900 0 +2.0Excel- Present- lent invention example 11 Zn—Ni 12 45 0 100 900 0 +1.0Excel- Present- lent invention example 12 Zn—Ni 12 45 0 30 850 0 +0.5Excel- Present- lent invention example 13 Zn—Ni 12 45 0 30 950 0 +2.5Excel- Present- lent invention example 14 Zn—Ni 12 45 0 30 1000 0 +4.5Good Present- invention example 15 Zn—Ni 12 45 0 30 900 120 +2.5 Excel-Present- lent invention example 16 Zn—Ni 12 45 0 30 900 300 +4.0 GoodPresent- invention example 17 Zn—Ni 9 45 6 30 900 0 +8.5 Poor A largeCompara- amount tive of ZnO example formed

TABLE 3 Plating layer Heating conditions Oxidation resistance Kind Coat-Average Heating Hold- Change Oxida- Steel of Ni ing η-phase temperature-temper- ing in tion sheet compo- content weight content rising rateature time weight Judg- appear- Re- No. nent (mass %) (g/m²) (mass %) (°C./s) (° C.) (s) (g/m²) ment ance marks 18 Zn—Ni 26 45 0 30 900 0 +6.0Poor A large Compara- amount tive of ZnO example formed 19 Zn—Ni 12 9 030 900 0 +7.5 Poor A large Compara- amount tive of scale example formed20 GI — 45 — 30 900 0 +9.0 Poor A large Compara- amount tive of ZnOexample formed 21 GA — 45 — 30 900 0 +7.0 Poor A large Compara- amounttive of ZnO example formed 22 GF — 45 — 30 900 0 +8.5 Poor A largeCompara- amount tive of ZnO example formed

TABLE 4 Plating layer Heating conditions Oxidation resistance Kind Coat-Average Heating Hold- Change Oxida- Steel of Ni ing η-phase temperature-temper- ing in tion sheet compo- content weight content rising rateature time weight Judg- appear- Re- No. nent (mass %) (g/m²) (mass %) (°C./s) (° C.) (s) (g/m²) ment ance marks 23 GL — 45 — 30 900 0 −13.5 PoorA large Compara- amount tive of ZnO example formed and scat- tered 24NONE — — — 30 900 0 +35.0 Poor A large Compara- amount tive of scaleexample formed

The above-mentioned steel sheets were not actually worked by hot press.However, as mentioned above, because the oxidation resistance isinfluenced by the change in plating layer due to the heating before hotpressing, particularly by the behavior of Zn contained in the platinglayer, the oxidation resistance of the hot-pressed member can beevaluated by the results of these examples.

Second Example

The plating layers that were different in Ni content, coating weight,and η-phase content were formed on the surfaces of the base steel sheetssame as that of First Example in the same manner as the case of FirstExample. Thereafter, a composite (solid content ratio of 15% by mass)having at least one compound selected from the group consisting of aSi-containing compound, a Ti-containing compound, an Al-containingcompound, a Zr-containing compound, and a Si and Zr-containing compoundthat are described below, and the balance solvent was applied to theplating layers. Thereafter, the steel sheets were baked under thecondition that the maximum arriving temperature of the steel sheet was140° C. and, as illustrated in Tables 5 to 15, any of a Si-containingcompound layer, a Ti-containing compound layer, an Al-containingcompound layer, a Zr-containing compound layer, and a Si andZr-containing compound layer that were different in thickness was formedthus preparing the steel sheets Nos. 1 to 32. Tables 5 to 15 show thesteel sheets Nos. 1 to 32 thus prepared.

Here, as the Si-containing compound, the Ti-containing compound, theAl-containing compound, and the Zr-containing compound, the followingcompounds were used.

Silicone resin: KR-242A manufactured by Shin-Etsu Chemical Co., Ltd.

Lithium silicate: lithium silicate 45 manufactured by Nissan ChemicalIndustries, Ltd.

Colloidal silica: SNOWTEX OS manufactured by Nissan Chemical Industries,Ltd.

Silane coupling agent: KBE-403 manufactured by Shin-Etsu Chemical Co.,Ltd.

Titanium coupling agent: ORGATIX TA-22 manufactured by Matsumoto FineChemical Co., Ltd.

Lithium titanate: lithium titanate manufactured by Titan Kogyo, Ltd.

Sodium aluminate: NA-170 manufactured by Asahi Chemical Co., Ltd.

Aluminum coupling agent: PLENACT AL-M manufactured by AjinomotoFine-Techno Co., Inc.

Zirconium acetate: Zirconium acetate manufactured by SANEI KAKO Co.,Ltd.

Zirconium coupling agent: ORGATIX ZA-65 manufactured by Matsumoto FineChemical Co., Ltd.

Furthermore, a solvent used when the silicone resin was used as acompound was the thinner mixed of ethylene glycol monobutyl ether andnaphtha in the mass ratio of 55/45. Furthermore, a solvent used when asubstance other than the silicone resin was used as a compound was thedeionized water.

The steel sheets Nos. 1 to 32 obtained in such a manner and illustratedin Tables 5 to 15 have the plating layers and the compound layers formedon the surface thereof in the order given above. Each steel sheet washeated in the electric furnace or by direct energization under theheating condition specified in Tables 5 to 15 and, thereafter, cooled at50° C./s cooling rate in a state that each steel sheet was sandwichedbetween dies made of aluminum. Thereafter, oxidation resistance same asthe case of First Example and painting adhesion described below wereevaluated. Tables 5 to 15 list the composition of the plating layer, thecomposition of the compound layer, the heating condition, and evaluationresults of the oxidation resistance and painting adhesion for each steelsheet.

Painting adhesion: Samples were taken from the steel sheets after heattreatment and a chemical conversion treatment was applied to each sampleunder the standard condition with the use of PB-SX35 manufactured byNihon Parkerizing Co., Ltd. Thereafter, a coating film having athickness of 20 μm was formed on the surface of the sample byelectrodeposition coating under the baking condition of at 170° C. for20 minutes with the use of the electrodeposition paint GT-10HT Graymanufactured by Kansai Paint Co., Ltd. for preparing a test piece. Next,cuts penetrating to the surface of the base steel sheet were made on theprepared test-piece surface to which the chemical conversion treatmentand the electrodeposition coating were applied in a lattice pattern (10by 10 squares, 1 mm intervals) by using a cutter knife, and a cross cuttape peel test for testing the painting adhesion by sticking and peelingan adhesive tape was performed. The painting adhesion was evaluated onthe basis of the following criteria. When the result of the evaluationis “Excellent” or “Good”, the sample is excellent in painting adhesion.

Excellent: No peeling

Good: Peeled in 1 to 10 squares

Fair: Peeled in 11 to 30 squares

Poor: Peeled in 31 squares or more

As illustrated in Tables 5 to 15, in the examples of the presentinvention, it is evident that the steel sheet provided with the compoundlayer is excellent in painting adhesion as well as in oxidationresistance.

TABLE 5 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 1 Zn—Ni 12 45 0 Silicone 0.5 30900 0 +0.4 Excel- Excel- Present- resin lent lent invention example 2Zn—Ni 12 45 0 Lithium 0.5 30 900 0 +0.7 Excel- Excel- Present- silicatelent lent invention example 3 Zn—Ni 12 45 0 Col- 0.5 30 900 0 +0.9Excel- Excel- Present- loidal lent lent invention silica example

TABLE 6 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 4 Zn—Ni 12 45 0 Silane 0.5 30900 0 +0.5 Excel- Excel- Present- coupling lent lent invention agentexample 5 Zn—Ni 12 45 0 Titanium 0.5 30 900 0 +0.7 Excel- Excel-Present- coupling lent lent invention agent example 6 Zn—Ni 12 45 0Lithium 0.5 30 900 0 +0.8 Excel- Excel- Present- titanate lent lentinvention example

TABLE 7 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 7 Zn—Ni 12 45 0 Sodium 0.5 30900 0 +0.8 Excel- Excel- Present- alumi- lent lent invention nateexample 8 Zn—Ni 12 45 0 Aluminum 0.5 30 900 0 +0.9 Excel- Excel-Present- coupling lent lent invention agent example 9 Zn—Ni 12 45 0Zirconium 0.5 30 900 0 +0.6 Excel- Excel- Present- acetate lent lentinvention example

TABLE 8 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 10 Zn—Ni 12 45 0 Zirconium 0.530 900 0 +0.6 Excel- Excel- Present- coupling lent lent invention agentexample 11 Zn—Ni 12 45 0 Silane 0.5 30 900 0 +0.4 Excel- Excel- Present-coupling lent lent invention agent + example zirconium coupling agent 12Zn—Ni 12 45 0 — — 30 900 0 +1.5 Excel- Fair Present- lent inventionexample

TABLE 9 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 13 Zn—Ni 12 45 0 Silicone 0.1 30900 0 +1.2 Excel- Good Present- resin lent invention example 14 Zn—Ni 1245 0 Silicone 0.4 30 900 0 +0.8 Excel- Excel- Present- resin lent lentinvention example 15 Zn—Ni 12 45 0 Silicone 2.0 30 900 0 +0.4 Excel-Excel- Present- resin lent lent invention example

TABLE 10 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 16 Zn—Ni 12 45 0 Silicone 3.0 30900 0 +0.3 Excel- Good Present- resin lent invention example 17 Zn—Ni 1245 0 Silicone 4.0 30 900 0 +1.0 Excel- Good Present- resin lentinvention example

TABLE 11 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 18 Zn—Ni 10 45 1 Silicone 0.5 30900 0 +0.6 Excel- Good Present- resin lent invention example 19 Zn—Ni 1845 0 Silicone 0.5 30 900 0 +0.4 Excel- Excel- Present- resin lent lentinvention example 20 Zn—Ni 25 45 0 Silicone 0.5 30 900 0 +0.3 Excel-Excel- Present- resin lent lent invention example

TABLE 12 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 21 Zn—Ni 12 10 0 Silicone 0.5 30900 0 +0.8 Excel- Good Present- resin lent invention example 22 Zn—Ni 230 0 Silicone 0.5 30 900 0 +0.6 Excel- Excel- Present- resin lent lentinvention example 23 Zn—Ni 12 60 0 Silicone 0.5 30 900 0 +0.3 Excel-Excel- Present- resin lent lent invention example

TABLE 13 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 24 Zn—Ni 12 90 0 Silicone 0.5 30900 0 +0.2 Excel- Excel- Present- resin lent lent invention example 25Zn—Ni 12 45 0 Silicone 0.5 3 900 0 +0.9 Excel- Excel- Present- resinlent lent invention example 26 Zn—Ni 12 45 0 Silicone 0.5 10 900 0 +0.7Excel- Excel- Present- resin lent lent invention example

TABLE 14 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- η-phase compound layer Average HeatingHold- Change Paint- Steel of Ni ing content Thick- temperature- temper-ing in ing sheet compo- content weight (mass com- ness rising rate aturetime weight Judg- adhe- Re- No. nent (mass %) (g/m²) %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 27 Zn—Ni 12 45 0 Silicone 0.5100 900 0 +0.3 Excel- Good Present- resin lent invention example 28Zn—Ni 12 45 0 Silicone 0.5 30 850 0 +0.3 Excel- Excel- Present- resinlent lent invention example 29 Zn—Ni 12 45 0 Silicone 0.5 30 950 0 +0.8Excel- Excel- Present- resin lent lent invention example

TABLE 15 Si(Ti, Al, Zr)- Oxidation Plating layer containing Heatingconditions resistance Kind Coat- compound layer Average Heating Hold-Change Paint- Steel of Ni ing η-phase Thick- temperature- temper- ing ining sheet compo- content weight content com- ness rising rate ature timeweight Judg- adhe- Re- No. nent (mass %) (g/m²) (mass %) pound (μm) (°C./s) (° C.) (s) (g/m²) ment sion marks 30 Zn—Ni 12 45 0 Silicone 0.5 301000 0 +1.0 Excel- Excel- Present- resin lent lent invention example 31Zn—Ni 12 45 0 Silicone 0.5 30 900 120 +0.6 Excel- Excel- Present- resinlent lent invention example 32 Zn—Ni 12 45 0 Silicone 0.5 30 900 300+1.0 Excel- Excel- Present- resin lent lent invention example

In this example, each steel sheet was not actually worked by hot press.However, in the same manner as the case of oxidation resistance, thepainting adhesion of the hot-pressed member can be evaluated by theresults of this example.

The present invention is applicable to members such as suspensionmembers or body structural members of an automobile manufactured by hotpressing.

1. A steel sheet for hot pressing comprising: a base steel sheet; and aplating layer that is formed on a surface of the base steel sheet at acoating weight of 10 to 90 g/m² and contains 10 to 25% by mass of Ni andthe balance Zn with inevitable impurities.
 2. The steel sheet for hotpressing according to claim 1, further comprising at least one compoundlayer selected from the group consisting of Si-containing compoundlayer, Ti-containing compound layer, Al-containing compound layer, andZr-containing compound layer, wherein the compound layer is formed onthe plating layer.
 3. The steel sheet for hot pressing according toclaim 1, wherein the base steel sheet has component composition having0.15 to 0.5% by mass of C, 0.05 to 2.0% by mass of Si, 0.5 to 3% by massof Mn, 0.1% by mass or less of P, 0.05% by mass or less of S, 0.1% bymass or less of Al, 0.01% by mass or less of N, and the balance Fe withinevitable impurities.
 4. The steel sheet for hot pressing according toclaim 3, wherein the base steel sheet further contains at least onecomponent selected from the group consisting of 0.01 to 1% by mass ofCr, 0.2% by mass or less of Ti, and 0.0005 to 0.08% by mass of B.
 5. Thesteel sheet for hot pressing according to claim 3, wherein the basesteel sheet further contains 0.003 to 0.03% by mass of Sb.
 6. A methodfor manufacturing a hot-pressed member, the method comprising: heatingthe steel sheet for hot pressing according to claim 1 to a temperatureranging from Ac₃ transformation point to 1000° C.; and hot-pressing theheated steel sheet.
 7. The steel sheet for hot pressing according toclaim 2, wherein the base steel sheet has component composition having0.15 to 0.5% by mass of C, 0.05 to 2.0% by mass of Si, 0.5 to 3% by massof Mn, 0.1% by mass or less of P, 0.05% by mass or less of S, 0.1% bymass or less of Al, 0.01% by mass or less of N, and the balance Fe withinevitable impurities.
 8. The steel sheet for hot pressing according toclaim 7, wherein the base steel sheet further contains at least onecomponent selected from the group consisting of 0.01 to 1% by mass ofCr, 0.2% by mass or less of Ti, and 0.0005 to 0.08% by mass of B.
 9. Thesteel sheet for hot pressing according to claim 7, wherein the basesteel sheet further contains 0.003 to 0.03% by mass of Sb.
 10. The steelsheet for hot pressing according to claim 4, wherein the base steelsheet further contains 0.003 to 0.03% by mass of Sb.
 11. The steel sheetfor hot pressing according to claim 8, wherein the base steel sheetfurther contains 0.003 to 0.03% by mass of Sb.
 12. A method formanufacturing a hot-pressed member, the method comprising: heating thesteel sheet for hot pressing according to any one of claim 2 to atemperature ranging from Ac₃ transformation point to 1000° C.; andhot-pressing the heated steel sheet.
 13. A method for manufacturing ahot-pressed member, the method comprising: heating the steel sheet forhot pressing according to any one of claim 3 to a temperature rangingfrom Ac₃ transformation point to 1000° C.; and hot-pressing the heatedsteel sheet.
 14. A method for manufacturing a hot-pressed member, themethod comprising: heating the steel sheet for hot pressing according toany one of claim 7 to a temperature ranging from Ac₃ transformationpoint to 1000° C.; and hot-pressing the heated steel sheet.
 15. A methodfor manufacturing a hot-pressed member, the method comprising: heatingthe steel sheet for hot pressing according to any one of claim 4 to atemperature ranging from Ac₃ transformation point to 1000° C.; andhot-pressing the heated steel sheet.
 16. A method for manufacturing ahot-pressed member, the method comprising: heating the steel sheet forhot pressing according to any one of claim 8 to a temperature rangingfrom Ac₃ transformation point to 1000° C.; and hot-pressing the heatedsteel sheet.
 17. A method for manufacturing a hot-pressed member, themethod comprising: heating the steel sheet for hot pressing according toany one of claim 5 to a temperature ranging from Ac₃ transformationpoint to 1000° C.; and hot-pressing the heated steel sheet.
 18. A methodfor manufacturing a hot-pressed member, the method comprising: heatingthe steel sheet for hot pressing according to any one of claim 9 to atemperature ranging from Ac₃ transformation point to 1000° C.; andhot-pressing the heated steel sheet.
 19. A method for manufacturing ahot-pressed member, the method comprising: heating the steel sheet forhot pressing according to any one of claim 10 to a temperature rangingfrom Ac₃ transformation point to 1000° C.; and hot-pressing the heatedsteel sheet.
 20. A method for manufacturing a hot-pressed member, themethod comprising: heating the steel sheet for hot pressing according toany one of claim 11 to a temperature ranging from Ac₃ transformationpoint to 1000° C.; and hot-pressing the heated steel sheet.